1. Field of the Invention
The present invention relates to liquid-crystal displays able to operate with reduced power consumption.
2. Description of the Prior Art
A block diagram showing the structure of a conventional liquid-crystal display is shown in FIG. 6. The figure is a block diagram of a (1/3) bias liquid-crystal display drive circuit comprised of liquid-crystal display panel 1, liquid-crystal display driver 2, power supply 3, resistors 4 to 6, ground (GND) 7, and frame frequency generator circuit 20. Liquid-crystal display panel 1--a liquid-crystal panel with a structure to be described below--displays text and pictures by on or off action of liquid-crystal picture elements. In synchronization with the frame frequency signal FL from frame frequency generator circuit 20, liquid-crystal display driver 2 sends output signal COMO, S1 . . . Sn to liquid-crystal display panel 1, causing each liquid-crystal picture element to turn on or off. Positioned between power supply 3 and ground (GND) 7 are resistors R, which have the same resistance value and provide 3 voltage levels--VL1, VL2 and VL3--that are input to power supply terminals 2.sub.a to 2.sub.c of liquid-crystal display driver 2. Frame frequency generator circuit 20 outputs to liquid-crystal driver 2 frame frequency signal FL which determines whether each picture element is turned on or off.
As shown in FIG. 7, the liquid-crystal display panel described above consists of common electrode 1.sub.0, segment electrodes 1.sub.a2 to 1.sub.an which are provided as resistance to common electrode 1.sub.0, substrates 1.sub.c and 1.sub.d, and liquid-crystal 1.sub.b. The polarizing board and other items unnecessary to this explanation are not shown in FIG. 7 Signals COMO, S.sub.1, S.sub.2, S.sub.3, . . . S.sub.n (shown within parentheses in FIG. 7) from liquid-crystal display driver 2 are input to common electrode 1.sub.0 and segment electrodes 1.sub.a1, 1.sub.a2, 1.sub.a3 . . . 1.sub.an, respectively. When the signals from liquid-crystal display driver 2 are input, the picture element turns on if the voltage difference between segment electrodes or between each segment electrode and common electrode 1o is higher than the constant threshold level of liquid-crystal display panel 1, or turns off if it is lower.
FIG. 8 is a timing diagram showing the waveforms of (1/3) bias, quarter-time-division signals COMO S.sub.1, S.sub.2, S.sub.3, . . . S.sub.n output from liquid-crystal driver 2 to liquid-crystal display panel 1. In FIG. 8, FL is the frame frequency signal that is sent from frame frequency generator circuit 20 to liquid-crystal driver 2, and COMO, S.sub.1, S.sub.2, S.sub.3, . . . S.sub.n are signals sent from liquid-crystal driver 2 to con,non electrode lo and segment electrodes 1.sub.a1, 1.sub.a2, 1.sub.a3 . . . 1.sub.an of liquid-crystal display panel 1. Intervals T.sub.2, T.sub.3 and T.sub.6 are the on intervals of the picture elements, while intervals T.sub.1, T.sub.4 and T.sub.5 are the off intervals.
Next, the operation of a conventional liquid-crystal display is explained using FIGS. 6 to 8. Power supply voltage V.sub.L of power supply 3 is divided into three voltage levels through resistors R of the same resistance value, with V.sub.L3 =V.sub.L, V.sub.L2 =(2/3) V.sub.L, and V.sub.L1 =(1/3) V.sub.L. These voltages are then input to liquid-crystal driver 2 with power supply V.sub.L3 input to power supply input terminal 2.sub.a, power supply V.sub.L2 to power supply input terminal 2.sub.b, and power supply V.sub.L to power supply input terminal 20. Liquid-crystal driver 2 outputs the COMO, S.sub.1, S.sub.2, S.sub.3, . . . S.sub.n signals--in synchronization with the frame frequency signal FL from frame frequency generator circuit 20--to common electrode lo and segment electrodes 1.sub.a1, 1.sub.a2, 1.sub.a3 . . . 1.sub.an of liquid-crystal display panel 1 shown in FIG. 7. Picture elements of liquid-crystal display panel 1 are turned on during intervals T.sub.2, T.sub.3 and T.sub. 6, and turned off during intervals T.sub.1, T.sub.4 and T.sub.5, as shown in FIG. 8. During interval T.sub.2, the picture elements within the area comprised of common electrode 1.sub.0 and segment electrode 1.sub.an are turned on by the COMO and S.sub.n output signals. During interval T.sub.3, the picture elements within the area comprised of segment electrodes 1.sub.a1 and fan are turned on by the S.sub.1 and S.sub.n output signals. During interval T.sub.6, the picture elements within the area comprised of common electrode 1.sub.0 and segment electrodes S.sub.n are turned on by the COMO and S.sub.n output signals. In other words, looking at the voltage difference between the COMO and S.sub.n output signals during interval T.sub.2, the voltage difference between GND (earth) and V.sub.L3 is added to the voltage difference between common electrode 1.sub.0 and segment electrode 1.sub.an. At this time, if the threshold voltage is greater than (2/3) V.sub.L, the voltage difference V.sub.L between common electrode 1.sub.0 and segment electrode 1.sub.an becomes greater than the threshold voltage. As a result, the picture elements in the area comprised of common electrode 1.sub.0 and segment electrode 1.sub.an turn on. In the same way, during interval T.sub.3, the picture elements in the area comprised of segment electrode 1.sub.a1 and common electrode 1.sub.0 are turned on by voltage difference V.sub.L, and during interval T.sub.6, the picture elements within the area comprised by segment electrode 1.sub.0 and segment electrodes 1.sub.an are turned on by voltage difference V.sub.L. During intervals T.sub.1, T.sub.4, and T.sub.5, since the voltage difference of the signals added to that between segment electrodes or common electrodes falls below the (2/3) VL threshold level, the picture elements turn off.
Due to the structure of a conventional liquid-crystal display as described above, it is necessary to generate plural power voltages required for display in the liquid-crystal display circuit, and thus current must constantly flow through resistors in order to divide the power supply into various voltages in respect to ground (GND). This constant current flow causes the problem of higher power consumption.